The present invention relates generally to digital audio broadcasting (DAB) and other types of digital communication systems, and more particularly, to techniques for estimating the power of first adjacent analog FM interference in such digital audio broadcasting and other types of digital communication systems.
Proposed systems for providing digital audio broadcasting in the FM radio band are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, many broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, for example, transmitted in two side bands, one on either side of the analog FM host signal, using orthogonal frequency division multiplexing (OFDM) sub-carriers. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel.
In the United States, the frequency plan established by current FCC regulations separates each transmitting station in a geographical area by 800 KHz. Any transmitting stations in adjacent geographical areas, however, are separated from a local transmitting stations by only 200 KHz. Thus, a particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of a FM host carrier in an adjacent geographic area overlaps in frequency with a portion of a digital signal side band. Although first adjacent analog FM interference, when present, typically affects only one of the two digital side bands, it nonetheless represents a limiting factor on the performance of DAB systems. The presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmissions, even when one of the two side bands is free from interference.
The most challenging task for each IBOC receiver is to handle the interference from the analog first adjacent to the digital signal at the edge of the coverage zone. The power of the first analog FM interference can exceed the DAB signal by a margin of 19 dB at the edge of the coverage. The bins that are highly corrupted by the first adjacent analog FM interference can produce false metrics in the Viterbi decoder algorithm and as such substantially degrade the performance of the soft decoder. As far as the synchronization algorithm is concerned, the first adjacent analog FM interference is the dominant factor in the compound signal (DAB plus FM analog) in the time domain. Since the signal in the time domain is processed for the purpose of frame acquisition, the algorithm will fail as a result of the high power analog FM component. In any case, the receiver must know the power levels of the upper and lower first adjacent interferences in order to cope with the synchronization failure as well as high bit error rate (BER).
A need therefore exists for a method and apparatus for estimating the power of the first adjacent FM analog interference. A further need exists for a method and apparatus that estimate the number of OFDM bins that are corrupted by the first adjacent FM interference.
Generally, a method and apparatus are disclosed for estimating the power of the first adjacent FM interference and for using the estimated power to identify the number of OFDM bins that are at least partially corrupted by the first adjacent FM interference. The number of corrupted bins, Ncorr, is obtained based on measurements of the power of each of the first adjacent FM analog interference, Ik, and the power of the background noise, Nk, of the digital side bands. The power measurements, together with the known slope of the analog signal, is used to identify when the power of the measured background level equals the prorated power of the slope. All of the bins that fall below a threshold value are determined to be at least partially corrupted.
According to one aspect of the invention, the FFT output provides the power of the first adjacent FM analog interference, Ik, measured, for example, at xc2x1200 kHz, and the power of the background noise, Nk, of each of the digital side bands, measured, for example, at 130 kHz and xe2x88x92130 kHz. The receiver includes blocks for using the measured power values and the known slope of the first adjacent FM interference to determine the number of corrupted bins, Ncorr, in accordance with the following equation:                                           N            corr                    =                                    10              ⁢                              xe2x80x83                            ⁢              log              ⁢                              xe2x80x83                            ⁢                              (                                                      σ                    I                    2                                                        σ                    N                    2                                                  )                                                    Δ              ⁢                              xe2x80x83                            ⁢                              f                ·                                  35                  /                  100                                                                    ,                  
                ⁢        where                            (        1        )                                                      σ            I            2                    =                                    1                              N                +                1                                      ⁢                          xe2x80x83                        ⁢                                          ∑                                  m                  =                                      K                    -                    N                                                  K                            ⁢                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                      I                    m                                    "RightBracketingBar"                                2                                                    ,        and                            (        2        )                                          σ          N          2                =                              1                          N              +              1                                ⁢                      xe2x80x83                    ⁢                                    ∑                              m                =                                  K                  -                  N                                            K                        ⁢                          xe2x80x83                        ⁢                                                            "LeftBracketingBar"                                      N                    m                                    "RightBracketingBar"                                2                            .                                                          (        3        )            
In a hard-masking implementation, the corrupted bins are discarded (set to zero) and in a soft-masking implementation, the partially corrupted bins are scaled according to the interference slope before further processing, for example, by the Viterbi decoder.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.